A 62-year-old woman was admitted for evaluation of an incidentally discovered adrenal mass and hypertension. CT scan revealed a 7 cm mass in the right adrenal gland. After careful examination, the patient was diagnosed with subclinical hypercortisolism (SH). Adrenalectomy was performed. Histopathological examination showed an adrenocortical adenoma. Symptoms and signs of myasthenia gravis appeared 5 months later. CT of the chest showed a solid tissue mass in the mediastinum. The patient underwent a sternotomy with excision of the tumor, which histologically proved to be a type 2B thymoma. We describe a rare case of SH due to an incidentally discovered adrenocortical adenoma in a patient who manifested myasthenia gravis after surgical remission of the cortisol excess.